Polymeric materials are commonly used in the manufacture of solar cell modules. Specifically, polymeric materials are principally used as encapsulants for solar cells and as backskin materials in solar cell modules. For crystalline silicon solar cells, the cells can be encapsulated such that a transparent encapsulant is used between a transparent superstrate (usually glass) and the solar cell. In this case, a second layer of encapsulant, which may be pigmented, can be used between the solar cells and the backskin material. For thin film solar cell modules (e.g., amorphous silicon, cadmium telluride, or copper indium diselenide) a single layer of encapsulant is employed.
With the heightened interest in solar cell modules in architectural applications, there has developed an even greater need than ever before for materials which are used in solar cell modules that can show high resistance to thermal creep at temperatures as high as 90.degree. C. Such temperatures have been reached in some architectural applications. temperatures have been reached in some architectural applications.
As used herein, the term "thermal creep" refers to permanent deformation of a polymer effected over a period of time as a result of temperature. Thermal creep resistance, generally, is directly proportional to the melting temperature of a polymer. For materials with low melting temperatures, it is necessary to cross-link the materials to given them higher thermal creep resistance.
Polymeric materials commonly used as transparent encapsulants include thermoplastics such as ethylene vinyl acetate (EVA) and ionomers. EVA, the most commonly used material, is a co-polymer of vinyl acetate and ethylene. Ionomers are copolymers of ethylene and methacrylic acid with a salt added to neutralize them. Ionomers can be used alone or with metallocene polyethylene. Metallocene polyethylene can be a copolymer (or comonomer) of ethylene and hexene, butene, or octene. Encapsulant materials comprising both ionomers and metallocene polyethylene are described in a commonly owned U.S. patent application, entitled "Encapsulant Material for Solar Cell Module and Laminated Glass Applications, Ser. No. 08/899,512, filed Jul. 24, 1997.
These polymeric materials each address the thermal creep resistance problem in a different manner. For EVA, which has a rather low melting point, chemical cross-linking is used to provide thermal creep resistance. An organic peroxide is added to the EVA and cross-links it using the heat of a lamination process. A problem with this chemical cross-linking procedure is connected with the fact that total cross-linking is not fully achieved. Therefore, the peroxide used as the cross-linking agent is not completely used up during the process and excess peroxide remains in the laminated EVA. The remaining peroxide can promote oxidation and degradation of the EVA encapsulant. Also, the addition of some organic peroxides to the EVA sheet extrusion process require stringent temperature control to avoid premature cross-linking in the extruder chamber. This makes EVA with this peroxide addition difficult to rhanufacture into sheet. lonomers do not require chemical cross-linking agents. Instead, thermal creep resistance is provided by the built-in cross linking which the ionically bonded regions in the ionomers provide. Metallocene polyethylene can have melting temperatures of about 100.degree. C., anywhere from 5 to 15.degree. C. higher than ionomers and about 40.degree. C. higher than EVA which has not been cross-linked. Thus, they can exhibit better thermal creep resistance simply by virtue of their higher melting temperature. However, even higher creep resistance may be called for than presently exists for any of these encapsulants.
Polymeric materials commonly used as backskin layers include a Tedlar (i.e., a DuPont trade name for a type of polyvinyl difluoride) laminate, polyolefins and polyolefin mixtures. Backskin layers of solar cell modules require thermal creep resistance at temperatures considerably greater than 90.degree. C. to satisfy a certification test known as the Relative Thermal Index (RTI). Thermal creep resistance at temperatures at or above about 150.degree. C. are called for to satisfy the RTI test. The Tedlar laminate now widely used does satisfy the RTI requirement but it has other limitations. It is expensive, requires an additional edge seal and is thin, the entire laminate being about 0.010 inches thick and the Tedlar only about 0.002 inches thick.
Polymeric materials that can be used as backskin layers to form frameless modules should also exhibit thermoplastic properties during lamination, which is typically performed at temperatures on the order of 140.degree. C.-175.degree. C. One type of frameless solar cell module is described in commonly owned U.S. patent application, entitled "Solar Cell Modules with Improved Backskin and Methods for Forming Same, Ser. No. 08/671,415, filed on Jun. 27, 1996. For such modules, the backskin material must be capable of being softened, molded and formed during lamination (i.e., it must exhibit enough thermoplastic behavior to allow for this, while still exhibiting enough thermal creep resistance to satisfy RTI requirements).